Do Async Functions in JavaScript Always Return Promises?

Implicit Promises in Async Functions

In JavaScript, async functions are functions that can asynchronously await a promise to complete and return its value. However, a common misconception is that async functions implicitly return a promise for the value they return.

Understanding the Return Value

While it may appear that async functions implicitly return a promise, this is not entirely accurate. The return value of an async function can be one of two things:

• A non-promise value: When an async function explicitly returns a non-promise value, it automatically wraps that value in a promise.
• A promise: The return value of an async function that is itself a promise remains unchanged.

Example with Non-Promise Value:

async function increment(num) {
  return num + 1;
}

increment(3).then(num => console.log(num)); // Logs: 4

Here, we return a number from the async function. However, the output is retrieved through .then(), indicating that it is a promise that has been resolved to the value 4.

Explicit Promises:

If we explicitly return a promise from an async function, we receive a promise for that value, not a promise for a promise.

async function increment(num) {
  return Promise.resolve(num + 1);
}

increment(3).then(num => console.log(num)); // Logs: 4

Exceptional Cases:

• Empty Async Functions: Even if an async function has no explicit return statement, it still returns a promise for undefined.
• Await in Async Functions: Using await within an async function does not affect the type of the return value. It will still be wrapped in a promise if the returned value is not a promise.

Conclusion

Async functions in JavaScript implicitly wrap non-promise return values in a promise. However, if the return value is already a promise, it is left untouched. This behavior differs from traditional return statements but is consistent with the underlying principles of generators in ES6.

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